Catalytic Hydrocracking of Tar from Gasification of Straw

Abstract

The investigation covers “low temperature” catalytic tar conversion into light hydrocarbons in order to increase the heating value of the raw gas and to eliminate environmental problems with tarry condensate.

The gas is aimed for production of heat and electricity with gas engines or turbines.

The project, sponsored by the Danish Ministry of Energy and Elkraft, has involved catalytic hydrocracking experiments in the laboratories of the Danish Technological Institute and on site an updraft gasification plant (1 MW) at the Kyndby Power Plant, Denmark.

Laboratory experiments were carried out with various catalysts in the temperature range 300-500°C using a synthetic gas mixture containing 12% H2, 12% CO, 16% CO₂40% N2, 20% H₂0, 0.3% phenol and 10-300 ppm H₂5 at atmospheric pressure. The phenol content corresponds to 16 g synthetic tar per Nm³dry gas. Each experiment lasted for at least 100 hours in order to evaluate the stability of the catalytic system. The types of catalysts tested were Co/Mo, Ni/Mo, Ni/W, Pt and Pd.

The best catalysts for conversion of phenol into gases were found among the Ni/Mo and Pt-catalytic system which obtained nearly total conversion (99%) at 500°C and SV = 1000 Nm³/m³/h. The conversion is a first order reaction with respect to the total pressure.

Experiments with a selected Ni/Mo catalyst in the raw gas from the gasifier confirmed the results from the laboratory experiments. The raw gas contained approximately 50 g tar/m³. The rate constant for conversion of tar was approximately 3 times higher than the rate constant for conversion of phenol. The catalyst had a stable activity at a molar hydrogen/tar ratio higher than 5-10.